Self Healing Materials

Sneak peak at images inside excellent paper reviewing self repairing materials. Click on link for a deep dive!

I’ve recently become more obsessed with self healing materials, which I have learned in materials science speak is “autonomous material systems”. I like that fancy title, in one part for it’s scienceness factor, but mostly because it reminds us that materials are complex systems, not a static substance. To get started I’ve included the Wikipedia definition is here (which has an excellent overview of the general research):

Self-healing materials are a class of smart materials that have the structurally incorporated ability to repair damage caused by mechanical usage over time. The inspiration comes from biological systems, which have the ability to heal after being wounded.

Below I’ve collected a variety of case studies and a general overview of some of the principles. If you’re ready to dive into the meat of the science there is a paper available here that really shows off the research.

Autonomous Materials Systems

The best site for an overview of the science as well as mind blowing examples of the materials research  is the Autonomous Materials Systems website from the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign (whew that’s a mouthful).

The materials are incredible. Their research presents three main types of self healing materials;

• Microencapsulated Systems – material containing little capsules filled with a healing agent that bonds when in contact with catalysts also embedded in the material (see diagram below).
• Microvascular Systems – materials filled with capillaries filled with healing agents
• Mechanoresponsive Polymers – modifications made at the chemical level that control how a material responds under strain – simple example is changing colour before failure. I’ll admit I understand this kind the least.

Wikipedia has a more complex break down, but I think the above categories work well for me.

Image by Carl Hastrich - modified from Beckman Institute for Advanced Science and Technology: Pop a couple of capsules, connect to a catalyst and done! Simple as that.

The images of the embedded capsules are amazing, as you can see below:

Image from Beckman Institute of Advanced Science and Technology: A scanning electron microscope image shows the fracture plane of a self-healing material with a ruptured urea-formaldehyde microcapsule in a thermosetting matrix.

Self Healing Microelectronics

There are some great movies that show how the above technology could repair scratches and dings in paints – for that perfect auto finish. The real holy grail that appears to be emerging is the development of semiconductors capable to self repairing surprisingly frequent damage. In fact, according to a paper released by Scott R. White and Nancy R. Sottos, up to 80% of semiconductors fail during manufacture. Businessweek has a great article here describing their research, all be it with a cheeky opening line from Jay Leno of all things.

Microvascular Systems

Image by Janet Sinn Hanlon, UIUC: Cracks in a brittle coating are healed autonomously via a three-dimensional microvascular network embedded in the underlying substrate. The network contains a healing agent (red) which polymerizes after contacting the catalyst (purple) in the damaged regions.

Another variation on the theme is to create a material embedded with capillaries of healing material that can come in contact with embedded catalytic substances to trigger self healing bonding. It appears this process is a little more labour intensive in fabrication, but makes for some amazing diagrams.

Image by Dorothy Loudermilk, UIUC: The microvascular self-healing concept consists of an embedded microvascular network containing a fluid healing agent and dispersed catalyst particles. Crack damage intersects the network, allowing healing agent to fill the crack and react with the catalyst. Healing is repeatable due to the vascular nature of the healing agent supply.

Show me the goods!

While there don’t seem to be many tangible products available, beyond self repairing paints, concrete may be one of the first mass produced items. A quick search for self healing concrete reveals many researchers rushing to find an affordable solution with some great results. Read here for an overview of the research behind the impressive photo below.

Image by Nicole Casal Moore: Normal concrete would have failed long ago, but in this case the self healing micro structure allows many small cracks to dissipate force rather than one major failure.

I’m curious to know how the strain testing works. Is this analogous to an earthquake? Are we looking at the future building material to be sought after in earthquake prone areas?

Self Healing and the Future

Obviously non of the case studies begin to have the level of functionality that compete with natural materials, such as skin and bones, but it does suggest there is a lot happening in this topic. I’m curious what the primary application will be that would be the major success to drive things forward. Self healing car paint is obvious, and reducing the failure rate of semiconductors sounds interesting, but I’m curious how designers would think differently if it was available at the consumer plastics level. Do we need cameras and cell phones to self heal? I’m assuming the real value will lie at the infrastructure level; self healing pipes, linings for electric cables and sewage sound a little boring, but aging infrastructure in cities is a huge issue. Toronto is currently facing an estimated $1.7 billion repair bill for it’s rusted water pipes. That’s an enormous, if unlikely, opportunity for innovation.

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